Aqueous dental compositions having a pH of less than 7, i.e. acidic dental compositions, are known from the prior art. Such compositions are for example prepared in the form of universal etch or one-part self-etching, self-priming dental adhesive compositions which typically contain a mixture of polymerizable acidic compounds having at least one polymerizable double bond as well as an acidic group, further polymerizable compounds having at least one polymerizable double bond, and an initiator system in a suitable solvent.
The acidity of the mixture is adapted so that sufficient etching activity on dentin and enamel surfaces may be provided. However, a high acidity leads to complex stability problems due to the activation of chemical bonds of the functional components of the mixture. Specifically, ester bonds present in the polymerizable compounds may be solvolysed under acid catalysis. Moreover, the initiator system may be activated in the acidic medium leading to premature polymerization of the mixture.
As a result of the stability problems of such mixtures, the storage stability at room temperature of commercial one-part acidic dental compositions known from the prior art may be insufficient. Accordingly, conventional one-part acidic dental compositions must be stored under cooling in e.g. a refrigerator in order to avoid deterioration by solvolysis or polymerization.
EP-A 1 548 021 suggests one-part acidic dental compositions in the form of hydrolysis stable one-part self-etching, self-priming dental adhesive compositions containing specific monomers having improved resistance against hydrolysis under acidic conditions. In order to improve the stability of the composition, EP-A 1 548 021 suggests a stabilizer such as hydroquinone monomethylether, 2,6-di-tert.-butyl-p-cresol, tetramethyl piperidine N-oxyl radical and galvanoxyl radical.
EP-A 1 776 943 discloses one-part self-etching, self-priming dental adhesives having a pH of at most 2 which comprise an aqueous mixture containing a thermal polymerisation inhibitor in the form a 1,4-hydroquinone being substituted or unsubstituted at its benzene ring and optionally being etherified at one of its hydroxyl groups by means of a C1-C18 saturated hydrocarbon group. In particular, the specific inhibitors tert-butyl hydroquinone (TBHQ) and tert-butyl hydroxyanisol (BHA) are disclosed and tested in view of their ability to improve the stability of the dental adhesive composition compared with conventional inhibitors such as hydroquinone, hydroquinone monomethylether, bisphenol A and propyl gallate.
U.S. Pat. No. 5,320,886 A relates to a hydrophilic fluid crosslinking adhesive composition for dental application, which composition comprises hydrophilic monomers obtained as reaction products of a dianyhdride, a hydrophilic monomer and a reactive reagent. Furthermore, the composition may include water, a miscible volatile solvent, or a combination thereof. This document discloses long lists for each component of said adhesive composition, wherein among other inhibitors or stabilizers, 2,5-di-tertbutyl hydroquinone (DTBHQ) is listed. However, this document fails to disclose compositions having a pH of less than 7. Furthermore, the document is silent on stability problems upon storage. Rather, a stabilizer is primarily used during the synthesis of the hydrophilic monomers obtained from the starting materials dianyhdride, hydrophilic monomer and reactive reagent. For the purpose for use during this synthesis, 4-hydroxymethyl-2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-(dimethylamino) methyl-phenol are disclosed as particularly preferred stabilizers.
US 2006/0069181 A1 discloses a dental composition comprising 0.1 to 10 wt.-% of water, polymerizable compounds, a photoinitiator and a stabilizer. The stabilizer may be selected from a list including 2,5-di-tert-butyl hydroquinone.
US 2010/0197824 A1 discloses a non-aqueous dental compositions which may contain 2,5-di-tert-butyl hydroquinone as a stabilizer.
US 2011/0028589 A1 discloses a dental polymerizable composition in which at least a polymerizable monomer and a radical generator are contained in different parts of a two or three part composition. One part of the composition may comprise 2,5-di-tert-butyl hydroquinone as a polymerization inhibitor.
The present inventors found that acidic aqueous dental compositions known from the prior art are problematic in that the inhibitor used therein may give rise to discoloration problems upon storage and/or during photocuring under acidic conditions.